The linear polyethylene film market is mainly composed of two major types of film resins: linear low density polyethylene (LLDPE) and high density, high molecular weight polyethylene (HMWPE). LLDPE grades typically have a density of 0.913 to 0.928 g/cm3 and a narrow molecular weight distribution. Most of these resins are made with Ziegler catalysts, which often yield a polydispersity (Mw/Mn) of 3.5 to 5. However, a smaller segment of this group is made with metallocene catalysts, which typically produce a polydispersity of 2 to 4. Because polymers with a narrow molecular weight distribution have poor melt strength and are more difficult to extrude, LLDPE film resins typically have a relatively high melt index, such as 0.8 to 3 g/10 min, and are blown into films of about a 1-mil thickness with a wide die gap, near zero frostline height (also called “in the pocket”), and a low blow-up ratio. These conditions are usually referred to as “low density conditions” or “LLDPE conditions.”
In contrast, HMWPE grades have a high density, typically 0.948 to 0.955 g/cm3, and a broad molecular weight distribution, which permits easy processing at very high molecular weight. The molten polymer has excellent melt strength, and the high molecular weight results in improved physical properties, including toughness and tear resistance. Typically, these polymers have a high load melt index (HLMI) of 5 to 15 g/10 min, and a polydispersity of greater than 20. The broad molecular weight distribution and high molecular weight means that these films have higher melt strength and can be blown more easily, and line conditions known as “high density conditions” often are chosen, which are quite different from the “low density conditions” described above. Generally, the die gap is smaller, the blow-up ratio is greater, and the frostline height is quite high, producing a large expanded bubble during the film blowing process.
HMWPE film grades usually have a bimodal molecular weight distribution. Normally, a Ziegler catalyst is passed through two reaction zones to produce two narrow molecular weight distribution components, one of higher molecular weight and the other of lower molecular weight. When the branching is concentrated into the higher molecular weight component, film strength is usually improved. Thus, these polymers tend to have most of the branching in the long chains. In contrast, LLDPE film grades tend to have more branching (or at least an equal amount) in the lower molecular weight part of the molecular weight distribution.
The high density of HMWPE films means that they often have higher modulus and yield strength than LLDPE films. Accordingly, HMWPE films can be much less prone to sag and stretch when loaded, such as when formed into a plastic bag. However, the higher molecular weight and the different blowing conditions tend to introduce more orientation into HMWPE films, compared to LLDPE films. Higher orientation can produce unbalanced tear resistance. That is, resistance to tear in the transverse direction (TD) is much higher than LLDPE films, whereas resistance to tear in the machine direction (MD) generally is much worse. This can be an advantage, or a disadvantage, depending on the end-use application. Although the higher molecular weight of HMWPE resins tends to increase puncture resistance, the higher density tends to diminish it. Thus, puncture resistance can sometimes be comparable to LLDPE, although this depends on the choice of samples being compared.
In sum, it would be beneficial to produce LLDPE polymers having the broad molecular weight distribution and higher molecular weight normally associated with HMWPE film resins, but with densities (e.g., 0.910-0.926 g/cm3) normally associated with LLDPE film resins. Accordingly, it is to these ends that the present invention is directed.